Plasma heating of a substrate with subsequent high temperature etching

ABSTRACT

We have discovered a method of reducing the effect of material sputtered/etched during the preheating of a substrate. One embodiment of the method pertains to preheating a substrate which includes a metal-containing layer which is to be pattern etched subsequent to preheating. The method includes exposing the substrate to a preheating plasma which produces a deposit or residue during preheating which is more easily etched than said metal-containing layer during the subsequent plasma etching of said metal-containing layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to a method of plasma heating a substrateand then etching the substrate, where deposits generated during plasmaheating are removed so that control over a critical dimension of anetched feature is maintained.

2. Brief Description of the Background Art

Maintaining a substrate at a particular temperature during semiconductorprocessing frequently enables control of the critical dimension of afeature on the substrate. Processing may involve chemical vapordeposition (CVD), physical vapor deposition (PVD), or plasma etching,for example.

In a plasma etching process, a substrate is typically placed on anelectrostatic chuck in an etch reactor in which the substrate is exposedto an etchant plasma. The temperature of the substrate may be controlledusing a gas in contact with the backside of the substrate. When the gasin contact with the substrate is at a lower temperature than thesubstrate, so that it acts as a coolant, the substrate may be allowed toheat by a reduction of the pressure of the coolant gas. Specifically,when the pressure of the gas in contact with the backside of thesubstrate is reduced, the rate of heat removal is decreased, and thusthe temperature of the substrate is increased.

Other methods for heating a substrate to a desired temperature includeuse of a resistive heating element embedded in the electrostatic chuck.The heated chuck may be used in combination with a coolant gas to adjustthe substrate temperature.

One method for heating a substrate surface involves the use of ionbombardment of the substrate surface during semiconductor processing. RFcoupled or microwave energy may be used to produce a plasma which is thesource of ions which come in contact with the substrate. A biasing powermay be applied to the substrate to attract ions toward the substrate, toprovide a more rapid heating of the substrate. However, use of asubstrate bias to attract ions toward the substrate generally causessputtering of more malleable materials on the substrate surface, such asmetals. Thus, when a metal layer is being etched, for example, a portionof the metal layer is typically sputtered up onto the sidewalls of anoverlying patterned mask which is used to provide patterned etching ofthe metal layer. Subsequent removal of the sputtered material has provendifficult. In an attempt to reduce or avoid sputtering of surfacematerials during plasma heating of a substrate surface, various gaseshave been used as the plasma source gas, to lessen the sputteringeffect. Specifically, it has been recommended that the gas used to formthe plasma be composed of one or more gases including, for example,oxygen, argon, silane, silicon tetrafluoride, helium, neon, krypton,xenon, nitrogen, or mixtures thereof. Nitrogen gas is said to produceless satisfactory results, with the material being heated possiblyforming nitrides, similar to the manner in which oxides are producedwhen an oxygen plasma is used.

In some instances, it has been recommended that no substrate biasing beused, and that a lower heating rate be accepted. The use of a substrateheating plasma generated using only microwave energy is said to reducethe amount of sputtering of a substrate surface. For instance, when anRF bias is applied to a substrate having a SiO₂ surface, the sputterrate of the SiO₂ is said to be on the order of 1000 Å per minute, butwhen no substrate bias is applied, and the only energy applied ismicrowave energy used to produce the plasma, the sputter rate is said tobe reduced well over 50%.

Substrate temperature control is an important factor in the control ofcritical dimensions of a feature during plasma etching of the feature.For instance, during etch processing, the materials from a portion ofthe layer that is etched, as well as compounds formed by a combinationof the etchant gases and the layer materials, may coat the sides of thepatterned mask overlying the feature which is being etched, or may coatthe sides of the feature being etched and thereby reduce the size of theopening through which etching occurs. This may result in an increase inthe size of the feature produced during etching and may result in anetch profile which is different from the top to the bottom of the etchedfeature. Such growth of a feature dimension and variation in featuresidewall profile may be critical and may detrimentally affect thefunctionality of the features. By increasing the temperature of thesubstrate during processing, etch byproducts remain more volatile, andcontrol over etch sidewall profile, as well as growth of criticalfeature dimensions, may be achieved.

High substrate temperature etching is advantageous when the materialsbeing etched are either metal or metal-containing compounds which are oflow volatility, such as, for example, platinum, copper, iridium, iridiumdioxide, lead zirconium titanate, ruthenium, ruthenium dioxide, bariumstrontium titanate, and bismuth strontium tantalate.

In summary, although it is possible to reduce sputtering during plasmaheating of substrates by reducing substrate biasing, this substantiallyslows the heating process. The use of resistance heaters in theelectrostatic chuck under the substrate is expensive and decreases theresponse time when it is desired to stop heating or to cool thesubstrate. Accordingly, there remains a need for a substrate heatingmethod that provides rapid substrate heating while reducing the effectof material sputtered during the heating process on the criticaldimension and sidewall profile of an etched feature.

SUMMARY OF THE INVENTION

We have discovered a method of reducing the effect of materialsputtered/etched during the heating of a substrate.

One embodiment of the method pertains to preheating a substrate whichincludes a metal-containing layer which is to be pattern etchedsubsequent to preheating. The method includes exposing the substrate toa preheating plasma which produces a deposit or residue duringpreheating which is more easily etched than said metal-containing layerduring the subsequent plasma etching of said metal-containing layer.

In another embodiment, plasma heating of a substrate and subsequentetching of a metal-containing layer included in said substrate iscarried out while maintaining control over a critical dimension of afeature etched in the metal-containing layer. In particular, the methodincludes:

a) supplying a first plasma source gas to a process chamber containing asubstrate, wherein the first plasma source gas is used to generate aplasma which is used to preheat the said substrate, and wherein theplasma source gas contains at least one gas which is slightly reactivewith the metal-containing layer;

b) preheating the substrate to a temperature of at least 150° C. usingion bombardment from the first plasma;

c) supplying a second plasma source gas which generates a plasma used toetch the metal-containing layer; and

d) etching the metal-containing layer, wherein essentially all of aresidue generated during the preheating of the substrate is removedduring the etching of the metal-containing layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows an etched stack of layers (100) including a patterned hardmask layer (108) overlying a platinum-containing layer (106), whichoverlies a titanium nitride layer (104), which overlies a silicon oxidelayer (102). Residue material (110) produced by sputtering duringpreheating of the substrate and during patterned etching of the stack ofthe layers (100) resides on the sidewalls (112). FIG. 1B shows theetched stack of layers of FIG. 1A after treatment with a diluted HFsolution to remove patterned hard mask layer (108). The etched structureproduced (120) includes platinum-containing layer (106) overlyingtitanium nitride layer (104), which overlies silicon oxide layer (102).Sputtered material (110) has collapsed upon itself after removal ofresidual silicon oxide hard mask layer (108).

FIG. 2A shows an etched stack of layers (200) including a patternedsilicon oxide hard mask layer (208) overlying a platinum-containinglayer (206), which overlies a titanium nitride layer (204), whichoverlies a silicon oxide layer (202). No sputtered material remains onetched sidewalls (212) after patterned etching of the stack of thelayers (200).

FIG. 2B shows the etched stack of layers of FIG. 2A after treatment witha diluted HF solution to remove patterned hard mask layer (208). Theetched structure produced (220) includes platinum-containing layer (206)overlying titanium nitride layer (204), which overlies silicon oxidelayer (202). There was no residual sputtered material on the sidewalls(212) of etched silicon oxide hard masking layer (208). There was noapparent residual sputtered material on the sidewalls (214) of theetched platinum layer (206).

FIG. 3 shows the time-temperature correlation during a substratepreheating step, where the preheating plasma is formed from nitrogen gasand the substrate is a stack of layers of the kind described withreference to FIG. 1A, but prior to patterned etching.

FIG. 4A shows an etched stack of layers (400) including a patternedsilicon oxide hard mask layer (408) overlying a platinum-containinglayer (406), which overlies a titanium nitride layer (404), whichoverlies a silicon oxide layer (402). Residue material (410) produced bysputtering during preheating of the substrate and during patternedetching of the stack of the layers (400) resides on the hard masksidewalls (412), as well as on the sidewalls (414) ofplatinum-containing layer (406).

FIG. 4B shows the etched stack of layers of FIG. 4A after treatment witha diluted HF solution to remove patterned silicon oxide hard mask layer(408). The etched structure produced (420) includes platinum-containinglayer (406) overlying titanium nitride layer (404), which overliessilicon oxide layer (402). Residue material (410) has collapsed towardthe exterior of sidewalls (414).

FIG. 5A shows an etched stack of layers (500) including a patternedsilicon oxide hard mask layer (508) overlying aplatinum-containing layer(506), which overlies a titanium nitride layer (504), which overlies asilicon oxide layer (502). Residue material (510) produced by sputteringduring preheating of the substrate and during patterned etching of thestack of the layers (500) resides on the hard mask sidewalls (512), aswell as on the sidewalls (514) of platinum-containing layer (506). Thereis so much residual material (510) that it even covers the upper surface(516) of hard mask layer (508).

FIG. 5B shows an etched stack of layers (520) produced without asubstrate preheating step. The layer stack was the same as that of FIG.5A, and the etchant used during plasma etching of theplatinum-containing layer was the same as the etchant used to etch thelayer stack shown in FIG. 5A. No residual material was observed on theexterior surfaces of patterned silicon oxide hard mask layer (508)sidewall (512), nor on the exterior surfaces of etchedplatinum-containing layer (506) sidewall (514).

FIG. 6 shows an example of an apparatus which can be used to carry outthe etching processes described herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Described in detail below is a method of reducing the effect on etchedfeature critical dimensions of material sputtered during the heating ofa substrate. As a preface to the detailed description, it should benoted that, as used in this specification and the appended claims, thesingular forms “a”, “an”, and “the” include plural referents, unless thecontext clearly dictates otherwise.

We have discovered a method of reducing the effect of materialsputtered/etched during the heating of a substrate. This permits the useof ion bombardment to heat substrates despite the fact that the ionbombardment may sputter/etch features on the substrate causing depositsand residue to form on features within the substrate.

It is crucial in the production of submicron-sized devices that criticaldimensions be maintained during the etching of a semiconductor feature.When the feature to be etched requires high temperature etch conditions,it is necessary to preheat the substrate before beginning the featureetching, so that etch by-products are sufficiently volatile. Ionbombardment heating of a substrate which leads to sputtering/etching ofan exposed layer which is to be etched is possible without affecting thecritical dimension of the etched feature if the plasma source gas usedfor heating enables the subsequent removal of substantially all of thesputtered/etched material generated during the preheating step. Thesputtered/etched residue from preheating is removed during the etch stepwhich follows the preheating of a substrate, for example.

To enable removal of a preheating sputtered/etched material residue, theplasma source gas used to generate the preheating plasma may provide aplasma which is slightly reactive with at least the exposed layer whichis to be subsequently etched. Frequently the exposed layer is a metal.For example, and not by way of limitation, in the etching ofplatinum-comprising material, iridium-comprising material, andruthenium-comprising material, the preheating plasma is slightlyreactive with the metal-comprising material.

It may also be advantageous to have the preheating plasma source gascontain a gas which is slightly reactive with the patterned maskingmaterial which overlies the layer exposed for etching. This is the casewhen the masking material is capable of providing reactive species whichreact with the material sputtered/etched from the exposed layer duringthe preheating step.

No single plasma source gas combination will produce the desired etchresults on all substrates. Thus, for a given material layer to bepattern etched, one skilled in the art should select a combination ofplasma source gases for the etch step which will provide an acceptableetch rate and etch selectivity with respect to substrate materialsadjacent to the material which is being pattern etched, and meet otherrequired etch criteria. Once the combination of gases to be used duringthe patterning etch step has been determined, the particular plasmasource gas to be used during preheating of the substrate is selected.The plasma source gas for substrate preheating may be a single gas or acombination of gases, but needs to include at least one gas which is atleast slightly reactive with the substrate material to be etched.Preferably, the plasma source gas for generation of the substratepreheating plasma contains at least one gas from the combination ofetchant plasma gases used in the subsequent etching of the substratematerial, as this simplifies processing requirements.

This inventive method provides a relatively quick way of heating asubstrate without using a resistance heater in an electrostatic chuck,thereby avoiding the added cost of such equipment and the undesiredeffects when needing to cool the substrate. Further, this inventivemethod is not focused on eliminating the sputtering of material during asubstrate preheating step, but rather is focused on removing thematerial that is sputtered during the preheating step during thepatterned etching step.

I. AN APPARATUS FOR PRACTICING THE INVENTION

The embodiment etch processes described herein were carried out in aCENTURA® Integrated Processing System, available from Applied Materials,Inc., of Santa Clara, Calif. The system is shown and described in U.S.Pat. No. 5,186,718, the disclosure of which is hereby incorporated byreference. Although the etch process chamber used in the Examplespresented herein is shown in schematic in FIG. 6, any of the etchprocessors available in the industry should be able to take advantage ofthe etch chemistry described herein, with some adjustment to otherprocess parameters. The equipment shown in schematic in FIG. 6 includesa Decoupled Plasma Source (DPS) of the kind described by Yan Ye et al.at the Proceedings of the Eleventh International Symposium of PlasmaProcessing (May 7, 1996) and published in the Electrochemical SocietyProceedings (Volume 96-12, pp. 222-233, 1996), which is herebyincorporated by reference. The plasma processing chamber enables theprocessing of an 8 inch (200 mm) diameter wafer.

FIG. 6 shows a schematic of a side view of an individual CENTURA® DPS™metal etch chamber 600. The etch chamber 600 consists of an upperchamber 604 having a ceramic dome 606, and a lower chamber 608. Thelower chamber 608 includes a monopolar electrostatic chuck (ESC) cathode610. Gas is introduced into the chamber via gas injection nozzles 614for uniform gas distribution. Chamber pressure is controlled by aclosed-loop pressure control system (not shown) using a throttle valve618. During processing, a substrate 620 is introduced into the lowerchamber 608 through inlet 622. The substrate 620 is held in place bymeans of a static charge generated on the surface of electrostatic chuck(ESC) cathode 610 by applying a DC voltage to a conductive layer (notshown) located under a dielectric film (not shown) on the chuck surface.The cathode 610 and substrate 620 are then raised by means of a waferlift 624 and sealed against the upper chamber 604 in position forprocessing. Etch gases are introduced into the upper chamber 604 via gasinjection nozzles 614. The etch chamber 600 uses an inductively coupledplasma source power 626 and matching network 628 operating at about 2.0MHZ for generating and sustaining a high density plasma. The wafer isbiased with an RF source 630 and matching network 632 operating at about13.56 MHZ. Plasma source power 626 and substrate biasing means 630 arecontrolled by separate controllers (not shown).

II. THE METHOD OF THE INVENTION FOR PLASMA HEATING AND ETCHING ASUBSTRATE WHILE MAINTAINING CRITICAL DIMENSION

The present invention pertains to a method of reducing the effect onetched feature critical dimensions and etched sidewall profile, whichresults from the presence of residue produced during an ion-bombardmentheating of the substrate in which the feature resides. The sputteredmaterial from the preheating step is removed during the subsequentetching of the substrate.

A patterned hard mask layer may be used in plasma etching to coverportions of the underlying substrate layer, while leaving other portionsof the layer exposed to an etchant plasma. A hard masking layer istypically used for patterned etching of underlying layers when the layerto be etched is one which requires high temperature etching to obtain areasonable etch rate or selectivity, or where the etch by-products arenon-volatile and require a higher temperature substrate to reduce theamount of by-product which remains on the etched surfaces aftercompletion of the etch process. Many of the metal layers deposited toform conductive structures require the use of a patterned hard masklayer rather than a photoresist, due to the substrate temperaturerequired to produce the desired etch results. Examples, not by way oflimitation, of metals or metal-containing compounds which require a hightemperature substrate during etching include platinum, iridium, iridiumdioxide, ruthenium, and ruthenium dioxide. Typically these metals areused in the formation of capacitor electrodes, gate electrodes,contacts, and other conductive structures.

During the development of the present method, we performed numerousexperiments to determine the typical amount of material sputtered ontothe vertical sidewalls of a patterned hard masking layer during asubstrate (etch stack) preheating step. Table One, below, shows variousexamples of plasma source gases used to produce the preheating plasma,and a description of the appearance of the sidewall profile of thepatterned hard mask after completion of the preheating step. The etchstack was as previously described with reference to FIG. 1, where thepatterned hard mask layer is a bi-layer, as described in detail below.

TABLE ONE Preheated Platinum-Containing Substrate - Hard Mask SidewallProfile Run # 1 2 3 4 5 N₂ Preheat 50 — 100 — 15 Gas (sccm) O₂ Preheat —— — 100 — Gas (sccm) Ar Preheat 50 100 — — 30 Gas (sccm) Cl₂ Preheat — —— — 120 Gas (sccm) Hardmask Essentially Slightly Essentially EssentiallyVery Sidewall Vertical Tapered Vertical Vertical Tapered Profile (Light(Moderate (Light (Light (Heavy Build-up) Build-up) Build-up) Build-up)Build-up)

Each substrate was preheated for about 45 seconds. Other preheatingparameters included a process chamber pressure of about 20 mTorr; an RFsubstrate bias power of about 500 W; an RF plasma source power of about1800 W; and a cathode temperature of about 80° C.

With reference to FIG. 1, from bottom to top of the etch stack, thebottom layer of silicon oxide (SiO_(x)) 102 was about 8000 Å thick, theoverlying barrier layer of titanium nitride (TiN) 104 was about 200 Åthick, the platinum (Pt) -comprising layer 106 was about 2500 Å thick,and the patterned hard mask layer 108 was a bi-layer (not shown as abi-layer in FIG. 1), where the bottom layer was about 300 Å of TiN, andthe top, overlying layer was about 5000 Å of TEOS produced silicon oxide(SiO_(x)). The underlying metal layer was platinum.

The term “hardmask sidewall profile”, as used in Table One, refers tothe cross-sectional profile of the hard mask and to the angle of thepatterned mask sidewall after a substrate preheating step, relative toan underlying horizontal substrate. When large amounts of materialsputter during plasma preheating of the etch stack (substrate), suchsputtered material tends to build-up at the base of the patterned hardmask, forming a wedge-shaped hard mask sidewall. This wedge shape tendsto continue downward during etching of an underlying metal layer,affecting the sidewall profile of the etched metal layer and thecritical dimensions of the etched metal features. With reference toTable One, the hard mask profiles were studied after a preheat step andprior to etching of an underlying metal layer in order to evaluate theamount of material deposited on the vertical sidewall of the patternedhard mask during the preheating step, when different plasma source gaseswere used during the preheating step.

The best hard mask sidewall profiles were obtained in Runs #1, #3, and#4. When the plasma source gas during substrate preheating was made upfrom a mixture of nitrogen and argon gas, with a high (50%)concentration of nitrogen, or was nitrogen, or oxygen, respectively, thehard mask profile was essentially vertical, indicating only a lightbuild-up of sputtered material. However, when argon gas, or acombination of nitrogen, argon, and chlorine gas, represented by Runs #2and #5, respectively, was used as the plasma source gas for generationof the plasma used to preheat the substrate, the profile wassignificantly tapered. Thus, a wedge-like vertical layer of sputteredmaterial was deposited on the hard mask sidewall. In Run #2, the plasmasource gas used during substrate preheating was essentially non-reactivewith both the exposed platinum layer and with the overlying siliconoxide hard masking material. In Run #5, nearly 73% of the plasma sourcegas used during substrate preheating was Cl₂, which is highly reactivewith the exposed platinum layer and which is moderately reactive withthe overlying silicon oxide hard masking material.

Experiments were then performed to determine the final etch profile foretch stacks in which the etch stack (substrate) was preheated using aplasma generated from one plasma source gas, and the platinum-containinglayer was pattern etched using a plasma generated from another plasmasource gas. The results of these experiments are presented in Table Two,below.

TABLE TWO Preheated and Plasma-Etched Platinum-Containing Layer-EtchProfile Run # 1 2 3 N₂ Preheat Gas (sccm) 100 — — O₂ Preheat Gas (sccm)— 50 100 Ar Preheat Gas (sccm) — 50 — Cl₂, Pt Etch Gas (sccm) 120 120120 N₂ Pt Etch Gas (sccm) 15 15 15 Ar, Pt Etch Gas (sccm) 30 30 30 EtchProfile Essentially Vertical Tapered Tapered

Each substrate was preheated for about 45 seconds. Further preheatingparameters include a chamber pressure of about 20 mTorr; a substratebias power of about 500 W; a plasma source power of about 1800 W; and acathode temperature of about 80° C. The etch stack was the same as thatdescribed above with reference to Table One.

Patterned etching of the platinum layer was carried out until theplatinum etch completion was detected by an optical endpoint detector,then an overetch was carried out for an additional 30 seconds. Thepressure in the etch chamber was about 20 mTorr; the substrate biaspower applied was about 275 W; the plasma source power was about 900 W;and the cathode (substrate pedestal) temperature was about 80° C. Aspreviously mentioned, the platinum-containing layer had a thickness ofabout 2500 Å.

The best etch profile was obtained in Run #1. Only nitrogen gas was usedas the plasma source gas during preheating of the substrate; nitrogengas was also contained in the source gas used for generating theplatinum etchant plasma. Virtually no residue remained on the hard maskat the completion of etching, and a vertical etch profile was obtained.However, when a combination of oxygen and argon gas was used as theplasma source gas during preheating of the substrate (Run #2), residuedid remain on the surfaces of the hard mask and etched platinum featureat the completion of the platinum etching. A tapered profile wasobserved. When only oxygen gas was used as the plasma source gas duringpreheating of the substrate (Run #3), a similar, but slightly thickerresidue was observed on the hard mask and etched platinum features.Again, a tapered profile was observed.

Applicants used the etch plasma source gas and etch process conditionsdescribed in Table Two to etch the platinum layer in the same etchstack, where the substrate was not preheated using plasma heating, butwas heated using a “hot” electrostatic chuck (heated by an embeddedresistive heater). No residue was observed on the hard mask surfaces,nor on the etched platinum surfaces. This indicates that the residue inRun #2 or Run #3 occurs when sputtered platinum-containing materialgenerated during a plasma preheating step is not removed during thesubsequent etching of the platinum layer.

Applicants concluded, based on the above experimentation, that once asatisfactory plasma source gas is developed for etching a metal layersuch as platinum, the plasma source gas used for plasma preheating ofthe substrate may advantageously contain a gas which is used in etchingthe metal layer. However, the preheating plasma overall reactivity withthe metal layer material must be controlled. If the reactivity isinsufficient, built up residue from the preheating step may be sodifficult to remove that it remains during the metal etch step, to causeproblems with etch profile. If the reactivity is too great, the amountof built up residue present after the preheating step may be such alarge quantity that it cannot be adequately removed during the metaletch step, again causing problems with the etch profile. As used herein,a “lightly reactive” or “slightly reactive” plasma refers to a plasmawhich provides sufficient reactivity with the exposed layer to be etched(one of the metals described above, for example) that the sputter/etchbyproduct produced during preheating of the substrate is more easilyetched than the pure exposed layer (metal) during the subsequent etch ofthe exposed layer.

For example, and not by way of limitation, in the etching of platinum,the slightly reactive plasma to be used in a substrate preheating stepshould be capable of etching a pure platinum layer at an etch rate of atleast about 200 Å per minute. For example, if nitrogen alone was used toplasma etch a platinum layer, a maximum platinum etch rate of about 290Å per minute would be expected. One skilled in the art, after readingapplicants' disclosure will understand that it is important to use aplasma source gas (which may be a combination of gases) which generatesa preheating plasma which is slightly reactive with the exposed layer tobe etched. As previously mentioned herein, the preheating plasma mayalso be slightly reactive with an overlying hardmasking layer, if thehardmasking material will produce reactive species which react with thematerial sputtered from the exposed layer during the preheating step,thereby generating a sputtered/etched residue which is more easilyremoved during the subsequent etch step.

Although the hard masking material described herein with reference tothe etching of platinum is silicon oxide, it is not applicants' intentto be limited to this hard masking material, as other metal containinghard masking materials such as TiN, SiN, TiO₂, or high temperatureorganic masking materials such as “α-C” polymers (high temperatureamorphous carbon-comprising materials) and “α-FC” polymers (hightemperature fluorocarbon materials) and FLARE™ (a polyarylene ether,available from Allied Signal, Advanced Microelectronic Materials,Sunnyvale, Calif.) are also contemplated.

In the above examples, minimum residue after platinum etching wasobserved when nitrogen was present in both the plasma source gas usedduring substrate preheating and the plasma source gas used duringplatinum etching. One might then believe that chlorine could be used inthe plasma source gas for preheating of the substrate; however, the datashow that when chlorine is used, at least in a substantial amount,during substrate preheating, large amounts of residue are produced onthe hard mask sidewalls. These large amounts of residue distort the hardmask sidewall surfaces (the mask sidewall profile), so that the openingsthrough which the metal can be etched do not enable etching of the metallayer to the desired critical dimensions (even if it were possible togradually remove the residue during etching of the metal layer). Thisindicates that when the plasma used for preheating the substrate ishighly reactive with the metal layer, this does not provide a goodoverall result in terms of the etched feature. Thus, the use of chlorineas the major component of a preheating plasma source gas is notadvisable. However, the presence of small amounts of highly reactivegases in a source gas, so that the plasma itself is only slightlyreactive with the exposed layer to be etched may be acceptable. As usedherein, the term “highly reactive” plasma, with respect to the etchingof platinum is a plasma which produces a platinum etch rate in excess ofabout 800 Å per minute. For example, if a plasma generated from a solelychlorine source gas was used to plasma etch a platinum layer, a platinumetch rate of at least about 800 Å per minute would be expected, usingthe other process conditions and the process apparatus described above.

In view of the data obtained for platinum, the preheating plasma shouldbe capable of etching the material which is to be pattern etched at anetch rate ranging between about 200 Å per minute and about 800 Å perminute, preferably at an etch rate ranging between about 250 Å perminute and about 600 Å per minute. One skilled in the art will be ableto adjust the plasma source gas compositions and overall processconditions in their particular apparatus to achieve a desired result fora given material to be pattern etched in view of the disclosure providedherein.

When a non-reactive gas, typically an inert gas such as helium, argon,krypton, or xenon, is used as the sole plasma source gas for substratepreheating, moderate amounts of residue remained after the subsequentmetal etch. When a combination of a non-reactive gas and a lightlyreactive gas (argon/nitrogen) was used as the plasma source gas forsubstrate preheating, light to moderate amounts of residue remainedafter the subsequent metal etch. The best results were obtained when thelightly reactive gas (nitrogen) was used as the sole plasma source gasfor substrate preheating. However, the preheating plasma source gas maycontain a limited amount of a gas which is essentially non-reactive, asa diluent within the plasma source gas. A minor amount ofexperimentation would be required to determine how much diluent gas canbe used and still provide a sputtered/etched residue which is moreeasily removed during the etch step than a pure residue of the exposedmaterial which is to be etched during the etch step.

The principles discussed above are best illustrated with reference tothe Figures provided herein. FIG. 1A shows the etch profile obtained foran etch stack (a substrate) including, from top to bottom, the bi-layerpatterned hard mask 108 previously described, overlying aplatinum-containing layer 106, which overlies a titanium nitride layer104, which overlies a silicon oxide layer 102. Residue material 110produced by sputtering during preheating of the substrate and duringpatterned etching of the stack of the layers 100 remains on thesidewalls 112. The substrate was preheated using a plasma generatedsolely from O₂. The platinum-containing layer 106 was etched using theCl₂/N₂/Ar plasma source described in Table Two. The process conditionswere as described previously, above.

FIG. 1B shows the substrate of FIG. 1A after being exposed to a dilutedHF solution (typically about a 6:1 ratio of H₂O:HF). From FIG. 1B, onecan see how the sputtered material 110 on the vertical sidewalls 112 ofthe patterned hard mask 108 collapsed on top of the etched platinumlayer 106 once the hard mask was removed by the diluted HF solution.

FIG. 2A shows the etch profile obtained for an etch stack (a substrate)including from top to bottom, the bi-layer patterned hard mask 208previously described, overlying a platinum-containing layer 206, whichoverlies a titanium nitride layer 204, which overlies a silicon oxidelayer 202. No residue material was observed on the sidewalls 212 of thehard mask 208, nor on the sidewalls 214 of the etchedplatinum-containing layer 206, after completion of etch of theplatinum-containing layer 206. The substrate was preheated using aplasma generated solely from N₂, which produced a slight sputteredbuild-up, as indicated for Run #3 in Table One. The platinum-containinglayer 206 was etched using the same Cl₂/N₂/Ar etchant plasma describedwith reference to FIG. 1A. At the completion of etch of theplatinum-containing layer 206, the slight build-up of sputtered materialfrom the substrate preheating step had been removed.

FIG. 2B shows the substrate of FIG. 2A after exposure to a diluted HFsolution. FIG. 2B clearly illustrates that virtually all of thesputtered material was removed during the platinum-containing layerplasma etch step.

FIG. 3 illustrates the heating rate for preheating of the etch stackdescribed above when nitrogen alone is used as the plasma source gas forthe preheating plasma. One skilled in the art will appreciate that thisis a competitive heating rate for plasma preheating of a substrate.

FIG. 4A shows the etch profile obtained for an etch stack (a substrate)including from top to bottom, the bi-layer patterned hard mask 408previously described, overlying a platinum-containing layer 406, whichoverlies a titanium nitride layer 404, which overlies a silicon oxidelayer 402. Residue material 410 produced by sputtering during preheatingof the substrate, and during patterned etching of the stack of thelayers 400 resides on the sidewalls 412 of hard mask 408 and on thesidewalls 414 of etched platinum-containing layer 406. The substrate waspreheated using a plasma generated from 50 sccm of Ar and 50 sccm of N₂,and is referenced in Table One as Run #1. The platinum-containing layer406 was etched using the Cl₂/N₂/Ar plasma source described in Table Two.The process conditions were as described previously, above.

FIG. 4B shows the substrate of FIG. 4A after exposure to a diluted HFsolution. From FIG. 4B, one can see how the sputtered material 410 whichwas on the vertical sidewalls 412 of the patterned hard mask 408collapsed about the exterior surfaces of etched platinum-containinglayer 406 once the hard mask was removed.

FIG. 5A shows the etch profile obtained for an etch stack (a substrate)including, from top to bottom, the bi-layer patterned hard mask 508previously described, overlying a platinum-containing layer 506, whichoverlies a titanium nitride layer 504, which overlies a silicon oxidelayer 502. Residue material 510 produced during preheating of thesubstrate, and during patterned etching of the stack of the layers 500,resides in very large amounts on the sidewalls 512 of hard mask 508 andon the sidewalls 514 of etched platinum-containing layer 506. Thesubstrate was preheated using a plasma generated from 120 sccm of Cl₂,30 sccm of Ar, and 15 sccm of N₂, and is referenced in Table One as Run#5. The platinum-containing layer 506 was etched using the Cl₂/N₂/Arplasma source described in Table Two. The process conditions were asdescribed previously, above.

FIG. 5B is representative of the comparative example in which the etchstack described with reference to FIG. 5A is preheated using anelectrostatic chuck having a resistive heater embedded therein. Therewas no plasma preheating of the substrate. The platinum-containing layer506 was etched using the Cl₂/N₂/Ar plasma source described in Table Two.The process conditions were as described previously, above. No residuewas present after etching of the platinum-containing layer 506. Thisclearly illustrates that the residue material 510 observed withreference to FIG. 5A was produced as a result of the sputtering/etchingof substrate etch stack material which occurred during the plasmapreheating of the substrate using a plasma generated from the Cl₂/N₂/Arplasma source gas.

Similar studies were made on a more limited basis for other etch stackscontaining metals or metal-containing compounds which are of lowvolatility, similar to platinum. In particular, substrates containingetch stacks including either iridium or ruthenium dioxide were preheatedand etched in a manner similar to that described above. Either nitrogenor oxygen was used to generate the plasma used to preheat the substrate.The etchant plasma used to etch iridium was generated from a plasmasource gas containing O₂/Cl₂/CF₄. The etchant plasma used to etchruthenium dioxide was generated from a plasma source gas containingO₂/Cl₂/Ar. Although applicants were able to obtain etched iridiumprofiles which appeared to be relatively free from sputteredmetal-containing material, the overall etch results in terms of etchrate, selectivity, and etch profile were less than desired. Work todevelop a satisfactory etchant plasma for etching iridium will have tobe done prior to determining the best plasma source gas for preheatingof the substrates.

The etching of a ruthenium dioxide-comprising etch stack provided moreencouraging results, which are presented below.

TABLE THREE Ruthenium Dioxide-Containing Substrate Layer-Etch ProfileRuns 1 2 N₂ Preheat Gas (sccm) 100 — O₂ Preheat Gas (sccm) — 100 EtchProfile Virtually Vertical Virtually Vertical

Each substrate was preheated for about 45 seconds. Other preheatingparameters included a chamber pressure of about 20 mTorr; a substratebias power of about 500 W; a plasma source power of about 1800 W; and acathode temperature of about 80° C.

Each ruthenium dioxide layer was plasma etched for about 60 seconds andthereafter exposed to a diluted HF solution. The etchant plasma wasgenerated from a plasma source gas comprising 320 sccm of O₂, 80 sccm ofCl₂, and 20 sccm of Ar. Other process parameters included an etchchamber pressure of about 36 mTorr; a substrate bias power of about 200W; a plasma source power of about 1500 W; and a cathode temperature ofabout 80° C.

In Runs #1 and #2, both plasma gases that were used to preheat thesubstrate (O₂ and N₂) produced similarly good results after the plasmaetch and HF solution exposure. Virtually all of the sputtered materialwas removed during the plasma etch step, thereby producing aresidue-free, essentially vertical etch profile.

The above described preferred embodiments are not intended to limit thescope of the present invention, as one skilled in the art can, in viewof the present disclosure expand such embodiments to correspond with thesubject matter of the invention claimed below.

We claim:
 1. A method of preheating a metal-containing substratecontaining a metal selected from the group consisting of platinum,iridium, ruthenium, and combinations thereof, where said substratesurface is etched at a temperature of at least 150° C., wherein saidmethod comprises exposing said substrate surface to a preheating plasmagenerated from a first plasma source gas which includes a slightlyreactive gas that is selected so that a compound deposit or residueformed during said preheating is more easily etched during a subsequentpattern etching step than said metal-containing layer, followed by saidsubsequent pattern etching step carried out using a second plasma sourcegas which is different from and more reactive with said metal-containinglayer than said first plasma source gas.
 2. The method of claim 1,wherein said metal-containing layer is a platinum-containing layer and afirst plasma source gas used to produce said preheating plasma includesnitrogen.
 3. The method of claim 2, wherein said platinum-containinglayer is platinum.
 4. The method of claim 2 or claim 3, wherein saidfirst plasma source gas is at least 50% by volume nitrogen.
 5. Themethod of claim 4, wherein said second plasma source gas used duringsubsequent plasma etching of said platinum-containing layer or saidplatinum layer is at least 15% by volume nitrogen.
 6. The method ofclaim 1, wherein said metal-containing layer is a ruthenium-containinglayer and said first plasma source gas used to produce said preheatingplasma includes a gas selected from the group consisting of nitrogen,oxygen, and combinations thereof.
 7. The method of claim 6, wherein saidruthenium-containing layer is ruthenium oxide.
 8. The method of claim 6,wherein said ruthenium-containing layer is ruthenium.
 9. The method ofclaim 7 or claim 8, wherein said first plasma source gas is at least 50%by volume nitrogen.
 10. The method of claim 9, wherein said first plasmasource gas is nitrogen.
 11. The method of claim 7 or claim 8, whereinsaid first plasma source gas is at least 50% or more oxygen by volume.12. The method of claim 11, wherein said first plasma source gas isoxygen.
 13. The method of claim 9, wherein a said second plasma sourcegas used during subsequent plasma etching of said ruthenium-containinglayer is at about 70% or more oxygen by volume.
 14. The method of claim10, wherein said second plasma source gas used during subsequent plasmaetching of said ruthenium-containing layer is about 70% or more oxygenby volume.
 15. The method of claim 11, wherein said second plasma sourcegas used during subsequent plasma etching of said ruthenium-containinglayer is at about 70% or more oxygen by volume.
 16. The method of claim12, wherein said second plasma source gas used during subsequent plasmaetching of said ruthenium-containing layer is about 70% or more oxygenby volume.
 17. The method of claim 1, wherein said metal-containinglayer is an iridium-containing layer and said first plasma source gasused to produce said preheating plasma includes a gas selected from thegroup consisting of nitrogen, oxygen, and combinations thereof.
 18. Themethod of claim 17, wherein said iridium-containing layer is iridiumoxide.
 19. The method of claim 17, wherein said iridium-containing layeris iridium.
 20. The method of claim 18 or claim 19, wherein said firstplasma source gas is at least 50% by volume nitrogen.
 21. The method ofclaim 20, wherein said first plasma source gas is nitrogen.
 22. Themethod of claim 18 or claim 19, wherein said first plasma source gas isabout 50% or more oxygen by volume.
 23. The method of claim 22, whereinsaid first plasma source gas is oxygen.
 24. The method of claim 20,wherein a said second plasma source gas used during subsequent plasmaetching of said iridium-containing layer is about 70% or more oxygen byvolume.
 25. The method of claim 21, wherein a said second plasma sourcegas used during subsequent plasma etching of said iridium-containinglayer is about 70% or more oxygen by volume.
 26. The method of claim 22,wherein a said second plasma source gas used during subsequent plasmaetching of said iridium-containing layer is about 70% or more oxygen byvolume.
 27. The method of claim 23, wherein said second plasma sourcegas used during subsequent plasma etching of said iridium-containinglayer is about 70% or more oxygen by volume.
 28. The method of claim 4,wherein said first nitrogen-comprising plasma source gas is nitrogen.29. The method of claim 17, wherein said second source gas includesoxygen.
 30. The method of claim 1, wherein said second plasma source gasincludes an inert, non-reactive gas selected from the group consistingof helium, neon, argon.